Low axial stiffness thrust bearing

ABSTRACT

A low axial stiffnesss thrust bearing is disclosed. The thrust bearing includes a housing having a generally cylindrical space formed therein and a generally cylindrical thrust collar located coaxially within the space. The axial length of the cylindrical space is sufficiently greater than the axial length of the thrust collar to permit the thrust collar to move axially within the cylindrical space over a predetermined distance and to insure that first and second fluid chambers are formed on opposite sides of the thrust collar when the thrust collar is located centrally within the cylindrical space. A fluid circuit supplies fluid under pressure to the first and second chambers in such a manner that the fluid attempts to maintain the thrust collar at a predetermined axial location within the space even when external thrust forces are placed on the thrust collar.

This application is a division of U.S. patent application Ser. No.164,071, filed June 30, 1980, and now matured into U.S. Pat. No.4,325,583 which issued on Apr. 20, 1982.

BACKGROUND OF THE INVENTION

The present invention relates to a low axial stiffness thrust bearingand more particularly to a thrust bearing which may be used incombination with a conventional thrust bearing on a single or coupledshaft system.

Prior art thrust bearings such as a tilting pad or tapered land bearing,have a high axial stiffness under load, generally measured in millionsof pounds per inch. A typical use of such system is to axially locatethe drive shaft of a turbine-generator set such as that illustratedschematically in FIG. 1. As shown by way of example in FIG. 1, the driveshaft 10 of a gas turbine 12 is coupled to the central shaft 14 of anelectric generator 16 by a clutch mechanism 18. The drive shaft 10 isradially supported by a pair of radial bearings 20 and the central shaft14 is supported by a pair of radial bearings 22. During normaloperation, clutch 18 couples shafts 10 and 14 such that gas turbine 12drives electric generator 16. In this mode, electric generator 16generates electric power which may be applied to a utility power grid orother electrical system. When clutch 18 is engaged, shafts 10 and 14operate as a single shaft which is axially aligned by a standardhydrodynamic thrust bearing 24 such as the tilting pad or tapered landtypes. As long as clutch 18 remains engaged, the single thrust bearing24 serves to axially align both drive shaft 10 and central shaft 14 withrespect to gas turbine 12 and electric generator 16, respectively.

In those applications wherein electric generator 16 is supplying autility power grid, it is often desireable to have clutch 18 disengagein order that generator 16 may run as a motor for electrical networkpower factor correction. In such cases, and in the absence of otherprovisions, the axial alignment of central shaft 14 is no longermaintained by thrust bearing 24 and shaft 14 is free to move axially dueto the magnetic forces which result from electric generator 16 beingelectrically "off-center." The simplest way to solve this problem wouldappear to be to place an additional thrust bearing between electricgenerator 16 and clutch 18. While such a thrust bearing would insurethat central shaft 14 remains axially aligned with respect to electricgenerator 16, the use of an additional standard thrust bearing isimpractical for the following reasons.

The interiors of the turbine and generator contain high heat sourcessuch as hot gases, steam or current carrying electrical conductors. Theexterior turbine-generator shells, base and foundation, however, are ina relatively cool ambient environment. Two points on the shaft separatedby a given axial distance will therefore separate more from thermalexpansion than two equally distant points on the base. Thus if astandard design thrust bearing were placed between generator 16 andclutch 18, as illustrated schematically in FIG. 3 by thrust bearing 46,the distance between the thrust collars 26 and 48 would increase duringa turbine-generator start-up. The axial distance between the twobase-mounted bearing support points of thrust bearings 24 and 46,however, would grow a lesser amount. This difference in axial thermalexpansion together with the extremely high stiffness of a standardhydrodynamic thrust bearing would create destructive thrust loads at thetwo thrust bearings. It is clear therefore that two standard thrustbearings cannot be used on a single or coupled shaft system in such asituation.

In order to avoid this problem, the prior art has designed complicatedclutch mechanisms which will enable thrust bearing 24 to absorb thethrust on central shaft 14 imparted by electric generator 16 during theintervals in which clutch 18 disengages shafts 10 and 14. A simplifiedschematic diagram of one such prior art clutch is illustrated in FIG. 2.As shown therein, the primary components of the clutch 18 is an axiallydisplaceable housing 28, a pair of thrust collars 30, 32 and slidingsleeve 34. The housing 28 is mounted in a stationary housing 36 in amanner which permits housing 28 to move in an axial direction butprevents the rotation of housing 28 about the axis of shafts 10 and 14.Thrust collars 30, 32 are coupled to shafts 10, 14, respectively, forrotation therewith. A projection 38 on each thrust collar 30, 32 isreceived in a corresponding recess 40 formed in housing 28 so as todefine respective thrust bearings.

The sliding sleeve 34 is located radially inward of thrust collars 30,32 and is slidable in the axial direction. A plurality of teeth 42 areformed about the outer perimeter of opposite ends of sliding sleeve 34and engage corresponding teeth 44 located on the inner periphery of theinner ends of thrust collars 30, 32. When clutch mechanism 18 isengaged, sliding sleeve 34 is in the position illustrated causing thrustcollars 30, 32 to rotate as a single unit. Additionally, any thrustforces placed on central shaft 14 by electric generator 16 aretransmitted to drive shaft 10 via axially slidable housing 28 due to theinter connection between thrust collars 30, 32 and housing 28. As aresult, any thrust forces placed on shaft 14 are absorbed by thrustbearing 24.

When clutch mechanism 18 is disengaged, sliding sleeve 34 is movedaxially to the right as viewed in FIG. 2 so as to disengage teeth 42,44. In this condition, thrust collars 30, 32 (and with them shafts 10,14) are free to rotate independently of one another. However, thrustforces placed on central shaft 14 are still transmitted to shaft 10 viaaxially slidable housing 28 due to the inter connection between thrustcollars 30, 32 and housing 28.

When the clutch mechanism 18 is disengaged and drive shaft 10 isstationary, any thrust load placed on central shaft 14 will ultimatelybe absorbed by the thrust bearing 24. Relative rotary sliding motion,however, will exist between the projection 38 on the thrust collar 30and the corresponding recess 40 in the housing 28. These relativelysliding surfaces of projection 38 and recess 40 comprise a thrustbearing which must be capable of transmitting the full thrust load fromrotating central shaft 14 to stationary drive shaft 10.

The foregoing arrangement provides for the absorption of thrust loadfrom central shaft 14 when the clutch 18 is either engaged ordisengaged. It also avoids destructive thrust loads arising from thermalexpansion differences. However, the clutch 18 must be designed toinclude a thrust bearing consisting of projection 38 and recess 40. Boththe clutch 18 and the thrust bearing designs are complicated andcompromised by their interdependence. Thus such a clutch mechanism islikely to require more frequent repair than if the thrust bearingfunction is separated from the clutch function.

BRIEF DESCRIPTION OF THE INVENTION

In order to overcome the foregoing drawbacks of the prior art clutches,the present invention provides a low stiffness thrust bearing exhibitinga relatively large axial float which serves to axially align generatorcentral shaft 14 when clutch 18 is disengaged yet is able to withstandthe forces created due to differences in the thermal expansion of shafts10 and 14 when clutch 18 is engaged.

In accordance with the foregoing, the thrust bearing of the presentinvention comprises:

a housing having a generally cylindrical space formed therein;

a generally cylindrical thrust collar located coaxially within saidcylindrical space, the axial length of said cylindrical space beingsignificantly greater than the axial length of said thrust collar suchthat said thrust may be moved axially within said cylindrical space overa predetermined distance and such that the first and second fluidchambers are formed opposite sides of said thrust collar when saidthrust collar is located centrally within said cylindrical space; and

fluid circuit means for supplying fluid under pressure to said first andsecond chambers in such a manner that said fluid attempts to maintainsaid thrust collar at a predetermined axial location within saidcylindrical space even when external thrust forces are placed on saidthrust collar.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings several embodiments which are presently preferred; it beingunderstood, however, that the invention is not limited to the precisearrangements and instrumentalities shown.

FIG. 1 is a schematic diagram of a prior art turbine-generator set.

FIG. 2 is a schematic diagram of a prior art clutch mechanism.

FIG. 3 is a schematic diagram of a generator-turbine set using thethrust bearing of the present invention.

FIG. 4 is a schematic diagram of a low axial stiffness hydrostaticthrust bearing constructed in accordance with the principles of thepresent invention.

FIG. 5 is a schematic diagram of the thrust bearing of FIG. 4 underthrust load.

FIG. 6 is a schematic diagram of a combined hydrostatic-hydrodynamicthrust bearing constructed in accordance with the principles of thepresent invention.

FIG. 7 is a view of the tapered land surfaces on the side wall of thebearing of FIG. 6 as illustrated along lines 7--7 of FIG. 6.

FIG. 8 is a cross-sectional view of the tapered land surfaces on theside wall of the bearing of FIG. 6 taken along lines 8--8 of FIG. 7.

FIG. 9 is a graph illustrating the bearing load capacity of thehydrostatic-hydrodynamic thrust bearing of FIG. 6 as a function of thedisplacement of the thrust collar from its axially centered position.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like numerals indicate likeelements, there is shown in FIG. 3 a schematic diagram of agenerator-turbine set utilizing a low axial stiffness thrust bearing ofthe present invention. While such a generator-turbine set is illustratedby way of example, it should be recognized that the thrust bearing ofthe present invention may be used in connection with any single orconnected shaft system.

The generator-turbine system illustrated in FIG. 3 is identical to thatillustrated in FIG. 1 with the addition of the low stiffness thrustbearing 46 of the present invention. The main attributes of thrustbearing 46 are its low axial stiffness and relatively large axial float.As a result of these features, thrust bearing 46 is able to accommodatevariations in the spacing between the thrust collars 26, 48 of thethrust bearings 24, 46 respectively, caused by thermal expansion.Additionally, when clutch 18 disengages shafts 10 and 14, thrust bearing46 will have sufficient stiffness to absorb normal thrust forces placedon central shaft 14 by electric generator 16.

A schematic diagram of a first embodiment of a low stiffness hydrostaticthrust bearing formed in accordance with the principles of the presentinvention is illustrated in FIG. 4. As shown in FIG. 4, thrust collar 48is housed in a generally cylindrical housing 50 which surrounds thecentral shaft 14 of the electric generator 16. A pair of fluid seals(not shown) are located between respective radially inward edges 51, 52of housing 50 and the central shaft 14 so as to prevent leakage of fluidlocated in fluid chambers 54, 56.

Fluid chamber 54 is defined between the side wall 68 of housing 50 andthe side wall 67 of thrust collar 48 while fluid chamber 56 is definedbetween the side wall 70 of housing 50 and the side wall 69 of thrustcollar 48. The volume of each fluid chamber 54, 56 varies as a functionof the axial location of thrust collar 48 with respect to housing 50. Inthe embodiment illustrated, the volume of the two fluid chambers 54, 56will be equal when thrust collar 48 is centrally located within housing50 as illustrated in FIG. 4. When thrust collar 48 is displaced asillustrated in FIG. 5, the volumes of these chambers will vary in aninverse manner. Fluid, such as oil, is pumped under pressure into eachof the fluid chambers 54, 56 by respective pumps 58, 60. Particularly,pump 58 pumps oil from an oil supply into chamber 54 via a plurality ofopenings 62 while pump 60 pumps oil from an appropriate oil supply intochamber 56 via a plurality of openings 63. A plurality of exit openings64 are spaced circumferentially about the radially outward end ofhousing 50 at a point midway between the opposite side walls 68, 70 ofhousing 50. Exit openings 64 are connected to fluid chambers 54, 56 viaa cylindrical space 66 formed between the outer diameter of thrustcollar 48 and the inner diameter of housing 50. The rate at which fluidleaves fluid chambers 54, 56 is determined by the distances L1, L2,respectively, between the fluid chambers 54, 56 and the exit openings 64as measured in the axial direction. When thrust collar 48 is centrallylocated within housing 50, the distances L1, L2 are equal and an equalamount of fluid flows out of both fluid chambers 54, 56.

Pumps 58, 60 supply oil to their respective fluid chambers 54, 56 withequal pressure. As a result, when thrust collar 48 is centrally located,the fluid flow into and out of fluid chambers 54, 56 is equal and thefluid in fluid chambers 54, 56 applies equal and opposite forces to sidewalls 67, 69, respectively, of thrust collar 48. These forces attempt tomaintain thrust collar 48 centrally located within housing 50 and resistany movement of thrust collar 48 from its central position. If thethrust collar 48 is moved off center as illustrated in FIG. 5, thedistances L1, L2 between fluid chambers 54, 56 and exit openings 64 varyas shown causing an increased resistance to the flow of fluid out offluid chamber 56 and a decreased resistance to the flow of fluid out offluid chamber 54. As a result, the fluid pressure in fluid chamber 56(and therefore the force on side wall 69) is increased while the fluidpressure in fluid chamber 54 (and therefore the force on side wall 67)is decreased. In the preferred embodiment, the pressure of the fluid ineach fluid chamber 54, 56 varies linearly with the displacement ofthrust collar 48 off center; the pressure in one fluid chamberincreasing as the other decreases. The net effect of this is to place abiasing force on thrust collar 48 in a direction which attempts toreturn thrust collar 48 to its central location and which increases asthe distance that the trust collar is off center increases. In thismanner thrust bearing 46 absorbs thrust forces placed on central shaft14 and attempts to maintain proper axial alignment of the shaft.

While the particular force applied to thrust collar 48 by the fluidlocated in fluid chambers 54, 56 should be selected as a function of theparticular application of thrust bearing 46, this force should be lowrelative to the axial stiffness of a standard thrust bearing such as thetilting pad or tapered land types. The particular force exerted by theoil on thrust collar 48 can be adjusted by varying the operation ofpumps 58, 60 and/or the dimensions of fluid chambers 54, 56, exitopening 64 and cylindrical space 66. In any case, the pressure of theoil in fluid chambers 54, 56 should be chosen such that thrust bearing46 will exhibit a sufficiently low axial stiffness to permit thrustbearing 46 to accommodate changes in the relative positions of thrustcollars 48 and 26 due to thermal expansion and other known variations.Additionally, the pressure of the fluid in fluid chambers 54, 56 shouldbe chosen to be sufficiently high to enable thrust bearing 46 to absorbthe normal thrust forces which will be placed on bearing 46 when centralshaft 14 is disengaged from standard thrust bearing 24.

In designing housing 50, the distance between side walls 68, 70 (whichdistance defines the axial float of thrust bearing 46) should besufficient to allow for the worst case of differential axial thermalexpansion between the two thrust bearings and their supports.Preferably, the axial float of the thrust bearing should be at least 0.5inches. In contrast, the axial float of a standard thrust bearing istypically only 0.01 to 0.02 inches.

Referring now to FIGS. 6 through 8, there is illustrated ahydrostatic-hydrodynamic thrust bearing 46' which is constructed inaccordance with the principles of the present invention. The primarydifference between thrust bearing 46 and thrust bearing 46' resides inthe provision of tapered land surfaces 74, 76 located on opposite sidewalls 68, 70 of housing 50. The tapered land surfaces 74, 76 have aconverging wedged shape as best illustrated in FIGS. 7 and 8. Theprovision of these surfaces creates a hydrodynamic film pressure in thearea adjacent tapered land surfaces 74, 76 when the thrust collar 48approaches either extreme of its permissible axial travel. The resultantbearing load capacity is illustrated in FIG. 9. As shown therein, thrustbearing 46 will operate as a hydrostatic bearing as long as both sidewalls 67, 69 of thrust collar 48 are at least x inches (typically a fewthousandths of an inch) from tapered land surfaces 74, 76 of housing 50.When either side wall 67, 69 of thrust collar 48 comes within x inchesof either tapered land surface 74, 76, a hydrodynamic film pressure iscreated between the side wall of collar 48 and the tapered land surface74 or 76 increasing the force on the side wall and causing thrustbearing 46' to operate in a hydrodynamic manner as shown.

The hydrodynamic-hydrostatic thrust bearing 46' illustrated in FIGS. 6through 8 is somewhat preferable to the hydrostatic thrust bearing 46 ofFIGS. 4 and 5 since it exhibits a substantially higher maximum loadcapacity than the simple hydrostatic thrust bearing 46. As a result ofthis feature, the thrust bearing 46' is capable of withstanding a muchhigher thrust load produced by electric generator 16 when the gasturbine 12 is disengaged from the generator 16. Additionally, when theturbine and generator shafts are coupled, the thrust bearing 46' iscapable of supplying a reserve load capacity. Particularly, when thethrust collar 48 approaches either tapered land surface 74, 76, thethrust bearing 46' carries a substantially larger share of the turbinethrust.

While the thrust bearing 46' is capable of absorbing substantial thrustforces when unusually high forces are placed on central shaft 14, itexhibits the desired low axial stiffness in all but the extremeoperations of the thrust bearing. As such, the thrust bearing 46' willexhibit sufficient axial float to prevent the thrust bearing 24 fromreceiving excessive thrust due to thermal expansion as described above.

Although a preferred embodiment of this invention has been described,many variations and modifications will now be apparent to those skilledin the art, and it is therefore preferred that the instant invention belimited not by the specific disclosure herein, but only by the appendingclaims.

What is claimed is:
 1. A turbine-generator set comprising:a gas turbine;said gas turbine including a drive shaft supported by radial bearings; agenerator; said generator including a central shaft supported by otherradial bearings; a clutch mechanism effective for coupling said driveshaft and said central shaft; a thrust bearing on said drive shafteffective for generally aligning said drive shaft; a low axial stiffnessthrust bearing on said central shaft; said low axial stiffness thrustbearing including means for permitting an axial float of said centralshaft sufficient to axially align said central shaft when said clutchmechanism is disengaged and to prevent the development of destructiveaxial force due to thermal expansion of said drive shaft and saidgenerator when said clutch mechanism is engaged; said low axialstiffness thrust bearing including a housing having a generallycylindrical space formed therein; a generally cylindrical thrust collarlocated coaxially within said cylindrical space, an axial length of saidcylindrical space being substantially greater than an axial length ofsaid thrust collar whereby first and second fluid chambers are disposedin said cylindrical space at opposite sides of said thrust collar, saidthrust collar being axially movable within said cylindrical space over apredetermined distance; and fluid circuit means for supplying fluidunder pressure to said first and said second fluid chambers; said fluidcircuit means including means for urging said thrust collar toward apredetermined axial location within said cylindrical space.
 2. Aturbine-generator set according to claim 1 wherein said means for urgingincludes means for varying said pressure in said first and second fluidchambers as a function of an axial location of said thrust collar.
 3. Aturbine-generator set according to claim 1 wherein said means for urgingis effective to place a first axial force on said thrust collar in afirst axial direction in relation to a first fluid pressure in saidfirst fluid chamber and wherein said means for urging is furthereffective to place a second axial force on said thrust collar in asecond opposite axial direction in relation to a second fluid pressurein said second fluid chamber.
 4. A turbine-generator set according toclaim 3 wherein said fluid circuit means includes means for varying saidfirst and second pressures in said first and second fluid chambers, andtherefore for varying axial forces on said thrust collar in said firstand second axial directions as a function of an axial location of saidthrust collar.
 5. A turbine-generator set according to claim 1 whereinsaid fluid circuit means comprises:pump means for pumping fluid underpressure into said first and second fluid chambers; fluid removal meansfor permitting fluid to leave said first and second fluid chambers; andsaid fluid removal means including means for varying a rate at whichfluid is permitted to leave said first fluid chamber inversely with arate at which fluid is permitted to leave said second fluid chamber, aparticular rate at which fluid is permitted to leave such respectivefluid chamber varying as a function of a position of said thrust collar.6. A turbine-generator set according to claim 3 wherein:a first axiallength of said first fluid chamber is defined between a first axial endof said thrust collar and a first axial end of said cylindrical spaceand a second axial length of said second fluid chamber is definedbetween a second axial end of said thrust collar and a second axial endof said cylindrical space; and said fluid pressure in said first andsecond fluid chambers varies as a function of the axial position of saidthrust collar as long as said first and second axial lengths remaingreater than a predetermined value.
 7. A tubine-generator setcomprising:a gas turbine; said gas turbine including a drive shaftsupported by radial bearings; a generator; said generator including acentral shaft supported by other radial bearings; a clutch mechanismeffective for coupling said drive shaft and said central shaft; a thrustbearing on said drive shaft effective for generally aligning said driveshaft; a low axial stiffness thrust bearing on said central shaft; andsaid low axial stiffness thrust bearing including means for permittingan axial float of said central shaft sufficient to axially align saidcentral shaft when said clutch mechanism is disengaged and to preventthe development of destructive axial force due to thermal expansion ofsaid drive shaft and said generator when said clutch mechanism isengaged.